VSE script for to add constant gaps between strips?
I turned your question into one of my example scripts.  I'll include it inline because that's how stackexchange works:

import bpy


def gap_strips(strips, gap):
    strips = sorted(strips, key=lambda strip: strip.frame_start)

    cleanup = []
    for i in range(1, len(strips)):
        st0 = strips[i-1]
        st1 = strips[i]
        cleanup.append( [ st1, st1.channel ])
        st1.frame_start = st0.frame_start + st0.frame_final_duration + gap
    # blender's anti-overlap code might change the channel.  put it back.
    for x in cleanup:
        x[0].channel = x[1]

def gap_selected_strips(gap):
    gap_strips( bpy.context.selected_editable_sequences , gap)


gap_selected_strips(15)